Answer the question on basis of the passage given below:

In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases.

Except glycine, all other naturally occurring \(\alpha \)- amino acids are optically active, since the \(\alpha \)-carbon atom is asymmetric. These exist both in 'D' and 'L' forms. Most naturally occurring amino acids have L-configuration. L-amino acids are represented by writing the \(NH_2\) group on the left hand side.

Identify the amino acids which is amophoteric in nature

The correct answer is option 4.

Amino acids, the building blocks of proteins, can be classified based on the nature of their side chains (R groups). This classification into acidic, basic, and amphoteric categories is crucial for understanding the chemical behavior of amino acids in different environments, including within proteins and in various physiological conditions.

Acidic Amino Acids

Acidic amino acids contain an additional carboxyl group (-COOH) in their side chain. This group can lose a proton (H⁺) and become negatively charged at physiological pH (approximately 7.4), making these amino acids acidic. The side chain carboxyl group has a pKa value typically around 4.0, meaning it readily donates a proton at physiological pH. These amino acids are negatively charged (as their carboxyl groups are deprotonated) at neutral pH. They are often involved in enzyme active sites, where they can act as proton donors.

Examples:

Aspartic acid (Asp, D): Has a side chain with the structure -CH₂COOH. When deprotonated, it carries a negative charge (-COO⁻).

Glutamic acid (Glu, E): Similar to aspartic acid, but with an additional CH₂ group, giving it the structure -CH₂CH₂COOH. It also carries a negative charge when deprotonated.

Basic Amino Acids

Basic amino acids contain an additional amino group (-NH₂) in their side chain, which can accept a proton, making these amino acids basic. The side chain amino group has a high pKa value (typically above 9.0), meaning it remains protonated and positively charged at physiological pH. These amino acids are positively charged at neutral pH due to the protonation of their amino groups. They are often involved in binding negatively charged molecules, such as DNA or phosphate groups, and in catalysis where they can act as proton acceptors.

Examples:

Lysine (Lys, K): Has a side chain with the structure -(CH₂)₄NH₂. The amino group (-NH₃⁺) is protonated at physiological pH.

Arginine (Arg, R): Contains a guanidinium group in its side chain, which is highly basic and carries a positive charge.

Histidine (His, H): Contains an imidazole group that can be positively charged depending on the pH. Histidine is unique because its pKa is around 6.0, close to physiological pH, allowing it to switch between protonated and unprotonated states, making it a common participant in enzyme active sites.

Amphoteric Amino Acids (Neutral Amino Acids)

Amphoteric amino acids can act as both acids and bases due to the presence of both a carboxyl group (-COOH) and an amino group (-NH₂) in their structure. In this context, amphoteric amino acids are those that have side chains that are neither acidic nor basic, making them neutral at physiological pH. The carboxyl group can lose a proton (making the molecule act as an acid), and the amino group can accept a proton (making it act as a base). At physiological pH, these amino acids typically exist as zwitterions, where the amino group is protonated (-NH₃⁺) and the carboxyl group is deprotonated (-COO⁻), resulting in an overall neutral charge. They do not have additional functional groups that ionize at physiological pH.

Examples:

Glycine (Gly, G): The simplest amino acid with just a hydrogen as its side chain, making it neutral.

Alanine (Ala, A): Contains a methyl group (-CH₃) as its side chain, which does not ionize.

Valine (Val, V), Leucine (Leu, L), Isoleucine (Ile, I): All have hydrophobic, non-ionizable side chains.

Phenylalanine (Phe, F): Has a phenyl group as its side chain, which is non-polar and non-ionizable.

Tyrosine (Tyr, Y): Contains a hydroxyl group on its aromatic ring, which can ionize, but at a much higher pH than physiological pH.

Tryptophan (Trp, W): Has an indole side chain that does not ionize at physiological pH.

Serine (Ser, S) and Threonine (Thr, T): Contain hydroxyl groups, which do not ionize at physiological pH.

Cysteine (Cys, C): Contains a thiol group (-SH), which can ionize but typically does not at physiological pH.

Methionine (Met, M): Contains a sulfur atom in its side chain, which does not ionize.

Asparagine (Asn, N) and Glutamine (Gln, Q): Contain amide groups that do not ionize at physiological pH.

Based on this knowledge if we look at the given options we can identify them as:

So the correct answer is option 4.